1. Field of the Invention
This invention relates to an optical detection system, for example for use in a surveillance system.
2. Discussion of Prior Art
Conventional surveillance systems use closed circuit television (CCTV) cameras, monitors and video recorders, which use the same standards (PAL, SECAM, NTSC, etc.) as are used in broadcast television applications. Because of the use of this well-developed technology, CCTV surveillance systems can be highly cost-effective. Acceptable frame refresh rates and image resolution can be achieved, without excessive signal bandwidth demands, at a reasonable cost.
It is desirable, however, in surveillance applications, to achieve the greatest possible image resolution, as this may be necessary, for example to be able to identify an individual under surveillance.
Conventionally, this can only be achieved using a single fixed camera by providing a zoom image, which necessarily means that the field of view of the camera is restricted, with obvious disadvantages. Therefore, alternative prior art systems include multiple fixed cameras, which obviously increase the cost and weight of the system, and mechanically driven single cameras. However, these latter systems are also considerably more expensive than using a fixed camera, while still having the disadvantage that, since they need to be relatively slow moving, parts of the required field of view are actually not under surveillance for relatively long periods of time, for example many seconds.
UK Patent Application No. GB 2 090 100 A describes an optical imaging system in which a liquid crystal spatial light modulator is used to image sub-areas of a total image with the sub-areas being transmitted one by one to a detector, thereby apparently increasing the spatial resolution. The system described therein requires a respective optical channel for each sub-area, with each channel having individual optical components to generate an image of each sub-area on the detector. The system would therefore be costly to manufacture. In addition, in order to accommodate the required optical components a periscope arrangement is required for each optical channel. Whilst such an arrangement might be achievable for a relatively low number of optical channels, for example four, it would become overly complex if the number of channels were to be increased.
European Patent Application No. EP 0 029 568 A describes an imaging system where a liquid crystal spatial light modulator is used to select a sub-area to be imaged on a detector. The system described therein incorporates a simple lens having the spatial light modulator at its image plane and a compound lens which focuses images formed at each of four spatial light modulator elements onto the detector. The spatial light modulator is positioned at the image plane of the simple lens and at the focal plane of the compound lens. In order for the described system to work, it would appear that the spatial light modulator has to include some form of light scatterer. Such an arrangement is not optically efficient since light would be scattered in directions other than those towards the compound lens.
The present invention derives from the realisation that the relatively high frame refresh rate, provided by a conventional CCTV camera so that realistic moving images of broadcast quality can be achieved, is unnecessary for most surveillance applications. In such applications, it may be acceptable to sacrifice the high frame refresh rate to achieve higher image resolution and/or a wider field of view. In accordance with preferred embodiments of the invention, this is achieved by the use of an optical device which provides multiple images sequentially to the camera.
The present invention provides an optical detection system comprising:
(i) an image detector having an image detector area;
(ii) a lens arrangement for focusing light from a scene onto said detector, and
(iii) an optical device for acting upon light passing to said detector, characterized in that:
(a) said lens arrangement comprises a lens having a single optical axis; and
(b) said optical device comprises an array of optical elements each for acting upon light passing through a respective portion of said lens wherein said optical elements form a plurality of optical channels for the transmission of respective images to said detector through said lens; and wherein each image substantially fills the whole of the image detector area.
The system of the invention may further comprise a selective shutter device for controlling the transmission of light to said detector, said shutter device comprising an array of shutter elements and a controller for controlling said array of shutter elements, each of said shutter elements being selectively openable and closable in response to a respective control signal from said controller; and wherein said optical elements and said respective shutter elements in combination form a plurality of optical channels for the transmission of light to said detector through said lens; and wherein said shutter device provides means for selecting at least one of said optical channels for the transmission therethrough of an image to said detector.
The selective shutter device may be a spatial light modulator which in a preferred embodiment is a liquid crystal spatial light modulator. A possible alternative to a liquid crystal spatial light modulator might be a mechanical shutter. It is likely that mechanical devices would have fewer shutter elements than a liquid crystal array. Preferably, the selective shutter device is positioned adjacent the lens.
The system of the invention may be arranged to selectively detect images from a plurality of directions, with control of the selective shutter controlling the look direction. To achieve this multi-directional capability, the optical device may be an array of reflecting elements, each arranged to receive light from a particular look direction. Such devices would be particularly suitable for use in surveillance applications where it is desirable to obtain images from a variety of directions.
Alternatively, the system may be configured to have a single look direction but to detect images over a range of focal distances. Such systems have applications in situations where the direction in which an object will appear is known but the separation between the system and the object is uncertain. Conventionally, such a situation would require the use of an automatically focusing camera or a camera with a sufficient depth of focus to accommodate such uncertainties at the expense of image resolution. To achieve the variable focal range capability, the optical device of the invention may comprise an array of lenslets, each having a respective focal length.
The system of the invention may be configured to provide spectral sensitivity. This capability may be achieved by a system in which the optical device is an array of filters, each filter having a respective transmission bandwidth.